As disclosed in FIG. 9 of JP,A 9-328784, for example, there is conventionally known a hydraulic drive system for a construction machine, which is applied to a construction machine such as a super-large-sized hydraulic excavator of a class having its own weight of 70 tons or more, in particular, the so-called backhoe type hydraulic excavator including a swing body swingably mounted on a lower travel structure and a multi-articulated front operating mechanism comprising a boom rotatably coupled to the swing body, an arm rotatably coupled to the boom, and a bucket rotatably coupled to the arm to be open rearward in a ground contact state.
Such a hydraulic drive system comprises two hydraulic pumps driven by a first prime mover; two hydraulic pumps driven by a second prime mover; a boom hydraulic cylinder, an arm hydraulic cylinder and a bucket hydraulic cylinder supplied with hydraulic fluids delivered from the four hydraulic pumps for driving the boom, the arm and the bucket, respectively; a first group of directional flow control valves including a boom directional flow control valve, an arm directional flow control valve and a bucket directional flow control valve for controlling respective flows of the hydraulic fluids supplied from two of the four hydraulic pumps to the boom hydraulic cylinder, the arm hydraulic cylinder and the bucket hydraulic cylinder; and a second group of directional flow control valves including a boom directional flow control valve, an arm directional flow control valve and a bucket directional flow control valve for controlling respective flows of the hydraulic fluids supplied from the other two of the four hydraulic pumps to the boom hydraulic cylinder, the arm hydraulic cylinder and the bucket hydraulic cylinder. Then, by joining the hydraulic fluids from both the first group of directional flow control valves and the second group of directional flow control valves together for each pair of the boom directional flow control valves, the arm directional flow control valves and the bucket directional flow control valve, and thereafter supplying the joined hydraulic fluids respectively to the boom hydraulic cylinder, the arm hydraulic cylinder and the bucket hydraulic cylinder (i.e., by supplying hydraulic fluids usually used in two systems covering from hydraulic excavator pumps to directional flow control valves in a joined manner), the hydraulic fluid can be supplied to each hydraulic cylinder at a large flow rate required for the operation of the super-large-sized machine.
To supply the hydraulic fluid under a very high pressure at a very large flow rate, main lines must be constructed of hoses, steel pipes or the likes having very large diameters. However, because hoses practically available from the market at present have a maximum diameter of about 2 inches, several (e.g., two or three) hoses must be laid side by side in practice to meet the requirement. Accordingly, an allowable capacity as the main lines is restricted as compared with the supply and drain flow rate required for a hydraulic actuator, and a relatively large pressure loss occurs in each of hoses constituting the main lines. Hence, a very large pressure loss is eventually generated in the whole of a hydraulic circuit of the super-large-sized machine having long lines formed of hoses, steel piles or the likes, flow control selector valves, etc. The pressure loss increases an energy loss and causes another problem that the operating speed of the hydraulic actuator reduces and the working efficiency deteriorates.
To cope with the problems mentioned above, as disclosed in FIGS. 1 and 2 of the above-cited JP,A 9-328784, for example, a hydraulic drive system for a construction machine is also already proposed in which the number of hoses and a total length of lines formed of steel pipes, etc. in a super-large-sized machine are cut to reduce a total pressure loss.
That prior-art drive system comprises two hydraulic pumps driven by a first prime mover; two hydraulic pumps driven by a second prime mover; a boom hydraulic cylinder, an arm hydraulic cylinder and a bucket hydraulic cylinder supplied with hydraulic fluids delivered from the four hydraulic pumps for driving the boom, the arm and the bucket, respectively; a boom directional flow control valve, an arm directional flow control valve and a bucket directional flow control valve for controlling respective flows of the hydraulic fluids supplied from two of the four hydraulic pumps to the boom hydraulic cylinder, the arm hydraulic cylinder and the bucket hydraulic cylinder; a pair of boom bottom-side inflow control valve and boom rod-side inflow control valve, a pair of arm bottom-side inflow control valve and arm rod-side inflow control valve, and a pair of bucket bottom-side inflow control valve and bucket rod-side inflow control valve for controlling respective flows of the hydraulic fluids supplied from the other two of the four hydraulic pumps to rod pushing-side chambers and rod drawing-side chambers of the boom hydraulic cylinder, the arm hydraulic cylinder and the bucket hydraulic cylinder without passing the boom directional flow control valve, the arm directional flow control valve and the bucket directional flow control valve; and a pair of boom rod-side outflow control valve and boom bottom-side outflow control valve, a pair of arm rod-side outflow control valve and arm bottom-side outflow control valve, and a pair of bucket rod-side outflow control valve and bucket bottom-side outflow control valve for controlling respective flows of the hydraulic fluids drained to a reservoir from the rod drawing-side chambers and the rod pushing-side chambers of the boom hydraulic cylinder, the arm hydraulic cylinder and the bucket hydraulic cylinder without passing the boom directional flow control valve, the arm directional flow control valve and the bucket directional flow control valve.
Then, for example, when performing boom-raising, arm-crowing and bucket-crowing operations, the hydraulic fluids are supplied from the first-mentioned two hydraulic pumps to the respective rod pushing-side chambers of the boom hydraulic cylinder, the arm hydraulic cylinder and the bucket hydraulic cylinder through the boom directional flow control valve, the arm directional flow control valve and the bucket directional flow control valve, and the hydraulic fluids from the other two hydraulic pumps are joined with the flows of the hydraulic fluids, which are supplied after having passed the respective directional flow control valves, through a separately provided common high-pressure line and then through the boom bottom-side inflow control valve, the arm bottom-side inflow control valve and the bucket bottom-side inflow control valve, which are disposed in respective lines branched from it, without passing the boom directional flow control valve, the arm directional flow control valve and the bucket directional flow control valve. The joined hydraulic fluids are supplied to the respective rod pushing-side chambers of the boom hydraulic cylinder, the arm hydraulic cylinder and the bucket hydraulic cylinder.
Also, when performing boom-lowering, arm-dumping and bucket-dumping operations, the hydraulic fluids are supplied from the first-mentioned two hydraulic pumps to the respective rod drawing-side chambers of the boom hydraulic cylinder, the arm hydraulic cylinder and the bucket hydraulic cylinder through the boom directional flow control valve, the arm directional flow control valve and the bucket directional flow control valve, and the hydraulic fluids from the other two hydraulic pumps are joined from the common high-pressure line with the flows of the hydraulic fluids, which are supplied after having passed the respective directional flow control valves, through the boom rod-side inflow control valve, the arm rod-side inflow control valve, and the bucket rod-side inflow control valve without passing the boom directional flow control valve, the arm directional flow control valve, and the bucket directional flow control valve. The joined hydraulic fluids are supplied to the respective rod drawing-side chambers of the boom hydraulic cylinder, the arm hydraulic cylinder and the bucket hydraulic cylinder.
Thus, by providing not only ordinary hydraulic fluid supply routes extending from the first-mentioned hydraulic pumps through the directional flow control valves, but also hydraulic fluid supply routes extending from the other two hydraulic pumps through the common high-pressure line without passing the directional flow control valves, the hydraulic fluid can be supplied to each hydraulic cylinder at a large flow rate required for the operation of the super-large-sized machine. Further, the number of hoses and the total length of lines formed of steel pipes, etc. in the super-large-sized machine can be cut and the total pressure loss can be reduced.